Clean up templates and CSS

- season.html.twig: remove dead empty column, drop redundant flex-row
- tab_overview.html.twig: extract Twig macro for confirm modals, fix duplicate aria-labelledby IDs
- tab_result.html.twig: remove dead comment, replace inline widths with CSS classes, simplify nested row/col forms to d-flex gap-1
- backoffice.scss: add col-result-xs/sm/md column width classes
- quiz.scss: replace broken display:grid + justify-self:center with flexbox centering

assets/styles/backoffice.scss

@@ -0,0 +1,3 @@
+.col-result-xs { width: 10%; }
+.col-result-sm { width: 15%; }
+.col-result-md { width: 20%; }

assets/styles/quiz.scss

@@ -1,14 +1,17 @@
-html, body {
+html {
     height: 100%;
+}
+
+body {
+    min-height: 100%;
     background-image: url("../img/background.png");
-    background-position: center center;
+    background-position: center;
     background-repeat: no-repeat;
     background-color: black;
     color: white;
-
-    display: grid;
-    align-items: center;
-    justify-self: center;
+    display: flex;
+    flex-direction: column;
+    justify-content: center;
 }
 
 .elimination-screen {